1. Field of the Invention
The present invention relates to a system for controlling a fuel cell which generates an electric power by an electrochemical reaction of hydrogen and oxygen and is applied to a family use generator, portable generator and mobile generators for an automobile and ship.
2. Description of the Related Art
The output of the fuel cell is lowered by a supply shortage of hydrogen and/or oxygen, by a blockade of a fuel (hydrogen) line and/or air (oxygen) line due to water flooding, or by a lowering of electrical conductivity of an electrolyte film due to its drying. Further, the fuel cell is degraded and its reliability is lowered, if it is operated under the above mentioned bad conditions.
Although its reliability is improved due to margins of operational conditions its operational efficiency is lowered by excessive supply of hydrogen and oxygen or water.
Therefore, in JP2000-208161A, such a parameter as fuel gas supply is controlled in order to hold a standard deviation of the fuel cell output voltage within a prescribed range.
However, the fuel cell as disclosed in JP2000-208161A has a disadvantage that factors causing the output deviation are not clearly decided. Accordingly, irrelevant factors are unnecessarily controlled, thereby lowering the operational efficiency of the fuel cell system.
Further, in JP2002-164065A, a cooling water flow direction is switched, thereby forming a prescribed humidity distribution along the built-up direction of individual cells and diagnosing the humidity condition of the fuel cell on the basis of the output voltages of the individual cells. Here, it is assumed that the relative humidity is the highest at the cooling water entrance, while it is the lowest at the exit, because the cooling water temperature is the lowest at the entrance, while it is the highest at the exit.
However, the cell temperature is in general higher at an intermediate portion of the cell than at the cooling water exit. This is because heat is radiated through cell container, while heat radiation is not easily caused at the intermediate portion. Further, the temperature distribution in the fuel cell is diversified by various conditions such as a cooling water temperature, ambient temperature and generated electric power.
Therefore, the fuel cell as disclosed in JP2002-164065A has a disadvantage that the inside humidity is not precisely determined, because the cooling water temperature does not always become highest at the cooling water exit.